基于C的sip协议栈，文档特别全，实现比较容易。,Nice sip stack.

ser作为一个高效、高度可配置的simple服务器，结合sip协议的特点采用rtpproxy来实现NAT穿越和负载均衡,ser as a highly efficient and highly configurable simple server, combining the characteristics of sip agreement rtpproxy used to achieve NAT and load balancing across

SIP开发参考资料：中国通信行业标准协议RF3261-SIP Developer Reference: Chinese telecommunications industry standard protocols RF3261